1. Field of the Invention
The present invention relates to a method for measuring an aberration of a projection optical system of an exposure apparatus used in a lithography process for manufacturing a device such as a semiconductor device or a liquid crystal display device, and an exposure apparatus which is capable of measuring the aberration using a detection unit included in the exposure apparatus.
2. Description of the Related Art
A device having a fine pattern such as a semiconductor device, a liquid crystal display device, or a thin-film magnetic head is manufactured using a photolithography technique. Conventionally, a projection exposure apparatus is used in manufacturing such a device. The projection exposure apparatus is used for transferring a pattern formed on an original plate such as a reticle or a photomask onto a substrate such as a wafer by a projection optical system. The projection exposure apparatus exposes the substrate such as a wafer after aligning a projection image of the pattern of the original plate, which is formed by the projection optical system, with a pattern formed in advance on the substrate, by an alignment detection unit mounted on the apparatus. The position alignment includes an alignment of a pattern of the original plate with a pattern of the substrate as well as focusing of the pattern.
There have been demands for a projection exposure apparatus capable of transferring a reticle pattern onto a wafer with high resolution as development of integrated circuit with finer design and higher density progresses. Since a minimum line width (resolution) which can be transferred by the projection exposure apparatus is proportional to a wavelength of light used for the exposure and in inverse proportion to the numerical aperture (NA) of the projection optical system, higher resolution can be achieved by reducing the wavelength of the light source.
Thus, the light source of recent photolithography is changing to krypton fluoride (KrF) excimer laser with wave length of about 248-nm or argon fluoride (ArF) excimer laser with wave length of about 193-nm in place of the g-line (wave length of about 436 nm) or the i-line (wave length of about 365 nm) produced by a conventional extra-high pressure mercury lamp. Further, F2 laser with wavelength of about 157 nm is being developed. In the future, an exposure apparatus employing extreme ultra violet (EUV) light is expected to achieve a wavelength of a few to a hundred nanometers.
In addition, an immersion exposure apparatus has been developed for further improvement in the resolution of the exposure apparatus. In the immersion exposure apparatus, at least a part of a space between the projection optical system and the wafer is filled with a liquid having a refraction index higher than 1 to increase the NA. To be more precise, the space between the wafer and an apical surface on the wafer side of the projection optical system is filled with a liquid having a refraction index close to that of a photoresist layer. In this way, effective numerical aperture of the projection optical system on the wafer side is increased and the resolution can be improved.
In this type of the immersion exposure apparatus, a pattern formed on a reticle is transferred onto a wafer with precision using a predetermined magnification (reduction ratio). Thus, it is important to employ a projection optical system with enhanced image forming capability and with reduced aberration. However, along with today's increasing demand for an integrated circuit with a finer pattern, a wide exposure area and high NA are requested to the projection optical system, and accordingly a correction of the aberration of the projection optical system is becoming evermore difficult.
While the request for reducing the aberration of such a projection optical system is becoming strong, a need for an exposure apparatus which is capable of measuring and adjusting aberration of the projection optical system mounted on the exposure apparatus is increasing. A change in the aberration occurs due to deterioration of projection lens across the ages, or a thermal effect during an exposure process. Accordingly, an exposure apparatus is required which allows a simple measurement of the aberration state of the projection optical system at regular intervals on the exposure apparatus. In other words, a precise lens adjustment which is in line with the actual state of usage is essential to a highly-integrated device. Such adjustment becomes possible when the aberration of the projection optical system is measured on the exposure apparatus.
However, in a conventional method used for obtaining an aberration of a projection optical system mounted on an exposure apparatus, a pattern is actually exposed and developed and a shift or a shape of the pattern is measured by a scanning electron microscope (SEM) or the like and then an amount of aberration is obtained by estimation. Thus, according to the conventional method, in obtaining the amount of aberration of the projection optical system, a pattern needs to be actually exposed, developed, and its pattern shift needs to be measured by a SEM or the like. Then, based on the obtained amount of aberration, the projection optical system is adjusted. Further, such a sequence has to be repeated again in adjusting the projection optical system. Such a technique is discussed, for example, in Japanese Patent Application Laid-Open No. 8-22951.
The above-described technique in which a pattern is exposed and developed, and the pattern shift or shape is measured in obtaining the amount of aberration, takes considerable amount of time, which has become a problem regarding throughput of the apparatus. Further, determination of a factor which affects the aberration in the resist process or the development process is difficult. Furthermore, the measured amount of aberration tends to vary depending on the operator.
On the other hand, the aberration of the projection optical system is also obtained by using an interferometer which can measure a wavefront aberration. The interferometer is generally used for inspecting the projection optical system at its manufacturing stage. However, it is not generally mounted on the exposure apparatus. This is because the exposure apparatus including the interferometer is larger in size and costs more.